Niobium-stabilized 14% chromium ferritic steel, and use of same in the automobile sector

ABSTRACT

A process for production of sheet-metal strip of niobium-stabilized 14% chromium ferritic steel, characterized in that certain steel is subjected to:  
     cold rolling of the hot sheet metal with or without preliminary annealing,  
     final annealing of the sheet-metal strip at a temperature of between 800° C. and 1100° C. for a duration of between 1 minute and 5 minutes and preferably at a temperature of about 1050° C. for a time of about 2 minutes. Steel and exhaust manifold.

FIELD OF THE INVENTION

[0001] The invention relates to a niobium-stabilized 14% chromium ferritic steel, and to use of same in the automobile sector.

[0002] 1. Discussion of the Background

[0003] The steels used for parts situated upstream from an exhaust system of a motor vehicle, the hot part of the system, must have both good resistance to oxidation and good creep resistance. Good formability is also necessary for manufacture of the manifold. The steels used for these hot parts are often either austenitic steels, which are relatively expensive and have poor oxidation resistance, although their formability is good, or bistabilized ferritic steels. The bistabilized ferritic steels have good oxidation resistance but are relatively difficult to form.

[0004] 2. Object of the Invention

[0005] One goal of the invention is to provide an economical ferritic steel which exhibits very good resistance to both creep and oxidation at temperatures up to 1000° C. as well as improved hardness for forming purposes.

DETAILED DESCRIPTION OF THE INVENTION

[0006] The object of the invention is realized with a process for producing a sheet-metal strip of niobium-stabilized 14% chromium ferritic steel, wherein steel with the following composition by weight based on total weight:

[0007] carbon≦0.02%

[0008] 0.002%≦nitrogen≦0.02%

[0009] 0.05%≦silicon≦1%

[0010] 0%<manganese≦1%

[0011] 0.2%≦niobium≦0.6%

[0012] 13.5%≦chromium≦16.5%

[0013] 0.02%≦molybdenum≦1.5%

[0014] 0%<copper<1.5%

[0015] 0%<nickel<0.2%

[0016] 0%<phosphorus<0.020%

[0017] 0%<sulfur<0.003%

[0018] 0.005%<tin<0.04%

[0019] impurities inherent to smelting and iron

[0020] wherein the content of niobium, carbon and nitrogen satisfy the relationship:

9.5<Nb/(C+N),

[0021] is subjected to:

[0022] reheating before hot rolling at a temperature of between 1150° C. and 1250° C. and preferably at about (±15° C.)1175° C.,

[0023] coiling at a temperature of between 600° C. and 800° C. and preferably of about (±15° C.) 600° C.,

[0024] cold rolling of the coil with or without preliminary annealing,

[0025] final annealing of the sheet-metal strip at a temperature of between 800° C. and 1100° C. for a duration of between 1 minute and 5 minutes and preferably at a temperature of about (±15° C.) 1050° C. for a time of about (±15 sec) 2 minutes.

[0026] The other optional characteristics of the invention are:

[0027] after final annealing or before use, the sheet metal is subjected to heat treatment at a temperature of between 800° C. and 1000 ° C. for a time of between 1 minute and 100 hours and preferably at a temperature of about (±15° C.) 850° C. for a time equal to or less than 30 minutes. The invention also relates to a niobium-stabilized 14% chromium ferritic steel comprising, consisting of, and consisting essentially of, iron and the following by weight based on total weight:

[0028] carbon≦0.02%

[0029] 0.002%≦nitrogen≦0.02%

[0030] 0.05%≦silicon≦1%

[0031] 0%<manganese≦1%

[0032] 0.2%≦niobium≦0.6%

[0033] 13.5%≦chromium≦16.5%

[0034] 0.02%≦molybdenum≦1.5%

[0035] 0%<copper≦1.5%

[0036] 0%<nickel≦0.2%

[0037] 0%<phosphorus≦0.020%

[0038] 0%<sulfur≦0.003%

[0039] 0.005%<tin≦0.04%

[0040] impurities inherent to smelting,

[0041] wherein the content of niobium, carbon and nitrogen satisfy the relationship:

9.5≦Nb/(C+N).

[0042] Other optional characteristics of the invention are:

[0043] the Nb content satisfies the relationship 0.1≦ΔNb ≦0.5, where ΔNb=Nb−7(C+N) and preferably 0.2≦ΔNb≦0.3

[0044] the contents of niobium, silicon and molybdenum satisfy the relationship: ΔNb/(Si+Mo)≦0.9

[0045] the contents by weight of niobium and tin satisfy the relationship:

ΔNb/Sn≦50

[0046] the contents of manganese and silicon satisfy the relationship:

Si/Mn 23 1.

[0047] The contents of niobium, titanium, zirconium and aluminum satisfy the relationship:

N/(Ti+Zr+Al)≧0.16

[0048] after heat treatment, the steel contains an intermetallic phase of Fe2Nb3 type with tetragonal structure at the grain boundaries.

[0049] The invention also relates to use of the ferritic steel sheet metal in the automobile sector, particularly for production of exhaust system manifolds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The invention will be well understood by referring to the description hereinafter and the attached figures.

[0051]Figs. 1A and 1B respectively exhibit the micrograph of a steel according to the invention (No. 1 in Tables I and II), and the micrograph of a comparison steel (No. 6 in Tables I and II) after heat treatment of each of the two steels, which have the same ΔNb of about 0.25%.

[0052]FIG. 2 exhibits the micrograph of a comparison steel (No. 9 in Tables I and II) with a relatively high ΔNb of about 0.43%, wherein intergranular precipitates of Fe2Nb type distributed in disordered manner are present after heat treatment.

[0053]FIG. 3 presents the mechanical hardness characteristics for a steel according to the invention (No. 1 in Tables I and II) and two comparison steels (Nos. 6 and 9 in Tables I and II), before and after heat treatment to induce formation of type Fe2Nb3 or Fe2Nb precipitates respectively.

[0054] The ferritic steels containing elements such as titanium, zirconium, aluminum and manganese as listed in the compositions of steels Nos. 5 to 9 in Tables I and II exhibit the Fe2Nb lava phase as intermetallic phase at all temperatures. For a value of ΔNb≦0.3%, the Fe2Nb lava phase is completely in solution at temperatures equal to or higher than 950° C., as shown in FIG. 1B. This explains the poor creep resistance behavior of these steels at or above 950° C.

[0055] Although the elements such as titanium zirconium or aluminum are to be avoided in the composition of the steel according to the invention, they nevertheless can be present in the composition in contents such as:

[0056] titanium≦0.01%

[0057] zirconium≦0.01%

[0058] aluminum≦0.1%

[0059] and preferably satisfy the relationship:

p N/(Ti+Zr+Al)≧0.16%

[0060] In the steels according to the invention in which the elements are present in the following contents: molybdenum between 0.02% and 1%, silicon between 0.05% and 1%, and tin between 0.005% and 0.04%, and in which the relationships ΔNb/(Si+Mo)≦0.9, Si/Mn≧1, ΔNb/Sn≦50 and N/(Ti+Zr+Al)≧0.16% are satisfied, we find the Fe2Nb lava phase only at low temperature, or in other words about 650° C. At higher temperatures, or in other words at or above 700° C., the type Fe2Nb3 quadratic phase is the only intermetallic phase observed. This phase has lower solubility than does the Fe2Nb lava phase. For a low ΔNb of 0.23%, a large proportion of Fe2Nb3 remains present even at 950° C., as can be seen on the micrograph of FIG. 1A. The presence of the Fe2Nb3 phase in substantial quantity at high temperature has the advantage of generating very good creep resistance and formability of the steels according to the invention.

[0061] The Fe2Nb lava phase is an intermetallic compound which, when it is present in a steel, precipitates in disordered intragranular form at the grain boundaries and does not sufficiently prevent grain-boundary displacement, and so the material is subject to creep. A large quantity of this intermetallic precipitate is necessary to improve the creep resistance.

[0062] The precipitation of the Fe2Nb3 phase at the grain boundaries ensures reduction of the hardness of the steel compared with a steel in which all the intermetallic precipitates have passed into solution or have precipitated in intragranular form (FIG. 3).

[0063] If the Si/Mn ratio does not satisfy the criterion of being greater than 1, the Fe2Nb3 intermetallic phase is still formed. However, the manganese increases the solubility of the Fe2Nb3 intermetallic phase and the formation, at high temperature, of a phase Z of CrNbN type in the grains. Thus the Fe2Nb3 intermetallic phase dissolves at 950° C. The steel has poor creep and oxidation resistance. The silicon compensates for this effect.

[0064] To ensure good formability and good creep resistance, which is manifested by a substantial quantity of intermetallic precipitates at the grain boundaries, there was performed, after final annealing or before use, a heat treatment at a temperature on the order of 900° C., preferably on the order of 850° C., for a relatively short period, less than or equal to 30 minutes. The heat treatment permits a very fine homogeneous precipitation of the Fe2Nb3 phase at the grain boundaries. These precipitates act as nucleation centers. They permit very homogeneous precipitation of the Fe2Nb3 phase at the grain boundaries at all temperatures higher than or equal to 750° C., and this is favorable for good creep resistance.

[0065] To improve the corrosion resistance, copper can be added in a moderate concentration, lower than or equal to 1.5%.

[0066] Table I presents the chemical analyses of the studied alloys. Alloys 1 to 4 are alloys according to the invention. Alloys 5 to 9 are comparison examples.

[0067] Table II presents the results for creep at 950° C. after 100 hours, for cyclic oxidation at 950° C. and 1000° C. after 200 hours, for hardness after final annealing and after heat treatment at 850° C. according to the invention, and for ΔNb, for the intermetallic type present at T>700° C. and for the presence or absence of intermetallic phases at 950° C. This table also indicates whether or not the relationships are satisfied by the elements of the listed compositions.

[0068] The compositions which satisfy all the relationships and which therefore exhibit the best characteristics in terms of creep, oxidation and hardness before and after heat treatment, in combination with the lowest ΔNb, are alloys 1 to 4 according to the invention.

[0069] French patent application No. 99 11257 filed Sep. 9, 1999 is incorporated herein by reference. TABLE I Steel No. Cr Mo Si Mn Al Ti Nb Zr C N Sn ΔNb Claimed Nb 1 14 0.02 0.5 0.2 — — 0.4 — 0.012 0.015 0.01 0.23 alloys NbMo 2 14 1 0.5 0.2 — — 0.4 — 0.012 0.015 0.01 0.23 NbSi 3 14 0.02 1 0.2 — — 0.4 — 0.012 0.015 0.01 0.23 NbSiMn 4 14 0.02 1 1 — — 0.4 — 0.012 0.015 0.01 0.23 NbMn 5 14 0.02 0.05 1 — — 0.4 — 0.012 0.015 0.001 0.23 Comparison NbTi 6 14 0.02 0.05 0.2 — 0.1 0.4 — 0.012 0.015 0.003 0.26 examples NbAl 7 14 0.02 0.05 0.2 1 — 0.4 — 0.012 0.015 0.004 0.31 NbZr 8 17 0.02 0.06 0.5 — — 0.4 0.45 0.016 0.016 0.002 0.39 F17TNb 9 17 0.02 0.06 0.5 — 0.14 0.5 0.016 0.016 0.002 0.43

[0070] TABLE II Hard- ness (HV1) after Relation- heat ship 1: Formed Pre- treat- N/(Ti + Relation- at sence Oxidation Oxidation ment ΔNb Zr + Al) ≧ ship 2: Relation- Relation- T > at Creep at at Hard- at Steel No. % 0.16% Si/Mn ≧ 1 ship 3: ship 700° C. 950° C. 950° C. 950° C. 1000° C. ness 850° C. Nb 1 0 23 • • • • Fe2Nb3 ▴ 5 X X 143 130 NbMo 2 0.23 • • • • Fe2Nb3 ▴ 2 X X 147 141 NbSi 3 0.23 • • • • Fe2Nb3 ▴ 2 X X 158 NbSiMn 4 0.23 • • • • Fe2Nb3 ▴ 4 X X 156 NbMn 5 0.23 • • ∘ Fe2Nb3 □ 20 ∘ ∘ 152 NbTi 6 0.26 ∘ • ∘ ∘ Fe2Nb □ 20 ∘ ∘ 148 150 NbAl 7 0.31 ∘ • ∘ ∘ Fe2Nb □ 41 X X 160 F17ZrNb 8 0.39 ∘ • ∘ ∘ Fe2Nb ▴ 11 X X 161 F17TNb 9 0.43 ∘ • ∘ ∘ Fe2Nb ▴ 9 X X 159 163 

1. A process for producing a sheet-metal strip of niobium-stabilized 14% chromium ferritic steel, comprising subjecting steel comprising iron and the following by weight based on total weight: carbon≦0.02% 0.002%≦nitrogen≦0.02% 0.05%≦silicon≦1% 0%<manganese≦1% 0.2%≦niobium≦0.6% 13.5%≦chromium≦16.5% 0.02%≦molybdenum≦1.5% 0%<copper≦1.5% 0%<nickel≦0.2% 0%<phosphorus≦0.020% 0%<sulfur≦0.003% 0.005%<tin≦0.04% impurities inherent to smelting, wherein the content of niobium, carbon and nitrogen satisfy the relationship: 9.5≦Nb/(C+N), to: reheating before hot rolling at a temperature of between 1150° C. and 1250° C., coiling at a temperature of between 600° C. and 800° C., cold rolling of the coil with or without preliminary annealing, final annealing of the sheet-metal strip at a temperature of between 800° C. and 1100° C. for a duration of between 1 minute and 5 minutes.
 2. A process according to claim 1, wherein, after final annealing or before use, the sheet metal strip is subjected to heat treatment at a temperature of between 800° C. and 1000° C. for a time of between 1 minute and 100 hours.
 3. A sheet of niobium-stabilized 14% chromium ferritic steel comprising iron and the following by weight based on total weight: carbon≦0.02% 0.002%≦nitrogen≦0.02% 0.05%≦silicon≦1% 0%<manganese≦1% 0.2%≦niobium≦0.6% 13.5%≦chromium≦16.5% 0.02%≦molybdenum≦1.5% 0%<copper≦1.5% 0%<nickel≦0.2% 0%<phosphorus≦0.020% 0%<sulfur≦0.003% 0.005%<tin≦0.04% impurities inherent to smelting, wherein the content of niobium, carbon and nitrogen satisfy the relationship: 9.5≦Nb/(C+N).
 4. A sheet of ferritic steel according to claim 3, wherein the Nb content satisfies the relationship 0.1≦ΔNb≦0.5, where ΔNb=Nb−7(C+N).
 5. A sheet of ferritic steel according to claim 3, wherein the contents of niobium, silicon and molybdenum satisfy the relationship: ΔNb/(Si+Mo)<0.9.
 6. A sheet of steel according to claim 3, wherein the contents by weight of niobium and tin satisfy the relationship: ΔNb/Sn≧50.
 7. A sheet of steel according to claim 3, wherein the contents by weight of silicon and manganese satisfy the relationship: Si/Mn≧1.
 8. A sheet of ferritic steel according to claim 3, wherein the contents of niobium, titanium, zirconium and aluminum satisfy the relationship: N/(Ti+Zr+Al)≧0.16.
 9. A sheet of ferritic steel according to claim 3, wherein, after heat treatment, the steel contains an Fe2Nb3 intermetallic compound of with a tetragonal structure at grain boundaries.
 10. The process of claim 1, comprising reheating at about 1175° C., coiling at about 600° C., and final annealing at about 1050° C. for about 2 minutes.
 11. The process of claim 2, wherein said heat treatment occurs at a temperature of about 850° C. for a time of equal to or less than 30 minutes.
 12. The sheet steel of claim 3, wherein the Nb content is 0.2≦Nb≦0.3.
 13. An exhaust system manifold comprising the sheet steel of claim
 3. 